ApoScreen? Annexin V Apoptosis Kit-FITC (SB Cat. No. 10010-02) employs a fluorescein-conjugated Annexin V (Annexin V-FITC) in concert with propidium iodide (PI) to evaluate subpopulations of cells undergoing apoptosis. During the early stages of apoptosis, cells begin to display phosphatidylserine (PS) on the outer cell membrane where it is readily detectable by staining the cells with Annexin V-FITC. As the plasma membrane becomes increasingly permeable during the later stages of apoptosis, PI can readily move across the cell membrane and bind to cellular DNA, providing a means for identifying those cells that have lost membrane integrity through mechanisms including necrosis. When cells are double stained with Annexin V-FITC and PI, three different cell populations may be observed - (i) live cells that do not stain with either Annexin V-FITC or PI; (ii) necrotic cells that stain with both reagents; and (iii) apoptotic cells that stain with Annexin V-FITC only. Analysis may be performed on any flow cytometer equipped with a single laser using excitation at 488 nm. Alternatively, the cells can be examined with a fluorescence microscope equipped with filters for discrimination of FITC and rhodamine/PI.